One of the most fascinating aspects in the formation of the heart is the very early development of the electrical patterning as can be registered by the ECG, i.e. registration of the rhythmic waves of depolarizing activity over the cardiac muscle. In the mature heart the conduction system is held responsible for the rhythmic excitations and contractions. However, in chicken embryos a sinusoidal type of ECG can already be derived from the linear heart tube stages at about two days of development onward, and less than one day later when chamber formation has just been initiated, an adult type of ECG can be monitored. Why certain areas of the heart tube do not develop into the working myocardium and contribute to the formation of the cardiac conduction system is one of the key questions of cardiac embryology. By our recent findings that the transcriptional repressors Tbx2 and Tbx3 repress the chamber-specific program of gene expression and chamber formation in transgenic animals over-expressing these factors, we begin to understand these processes. Detailed reconstructions of the developmental patterns of expression of Tbx3 during development have revealed that Tbx3 is expressed in those areas of the heart tube that do not become chamber, i.e. in the sinu-nodal region, internodal region, atrioventricular junction, atrioventricular bundle and bundle branches. These observations provide an embryonic basis why some areas in the heart are more prone to arrhythmias than others.